The present invention relates to battery chargers and methods of charging batteries of electrochemical cells. The invention is particularly well adapted to the new generation of batteries that are characterized by high storage capacity and power for unit weight. The individual cells are often the high-temperature, type without aqueous electrolyte that may be damaged if charged at too high a voltage. In such cells the structural components may enter into undesired electrochemical and corrosive reactions at voltages above the fullcharge voltages.
These high-temperature cells employ calcogens and metal chalcogenides including such as sulfur, iron sulfide, copper sulfide, cobalt sulfide and nickel sulfide as positive electrode materials and alkali metals, alkaline earth metals and alloys of these metals including such as sodium, lithium, lithium-aluminum, lithium-silicon, calcium, calcium-aluminum, calcium-magnesium, calcium-silicon as the negative electrode materials. Nonaqueous electrolytes including molten salts and porous oxides typically are used.
However, it will be understood that the present invention has application to any battery of cells in which it is desired to equalize charge at a set voltage level in the individual cells. This equalization of charge and voltage is of particular advantage in those cells employing nonaqueous and other electrolytes which do not provide the overcharge protection afforded by the electrolysis of water to form hydrogen gas.
Battery chargers for electrochemical cells that do not have inherent overcharge protection require close control of the charging voltage to prevent electrochemical degradation of the structural components within the individual cell. For example, in a cell using FeS as positive electrode material and iron or iron-base alloys in the current collector, the upper voltage level that can be applied to an individual cell without electrolytic degradation is about 1.6 volts. The open circuit voltage at full charge for the LiAl/FeS cell is about 1.3 volts but at least a small additional voltage must be available for providing the charging current. For the LiAl/FeS.sub.2 cell with molybdenum current collector, the corresponding voltages are about 2.1 and about 1.8 volts. Charge voltages must be controlled within these narrow ranges to permit full charge to each cell without an electrochemical attack by the electrolyte onto the cell structural components. It will be clear that with other electrochemical cell systems and other structural or current collector materials, the permissible voltage range may differ from these examples.
Batteries of these type cells will involve a plurality of series-connected cells possibly in parallel banks to obtain desired operating voltage and current levels for external loads. Traditional recharging methods of applying a constant voltage across the battery of cells or regulating the current flow through the battery can result in excessive voltage on some individual cells before others are fully charged. Cells near and above the fully charged state may be subjected to voltage levels that result in electrochemical degradation.
In other cells using aqueous electrolyte, overcharge protection is often afforded by the electrolysis of water to form hydrogen gas. Although this reaction protects such batteries including the conventional lead-acid batteries from damage resulting from overcharging, it involves a waste of electrical energy, increased terminal corrosion and the danger of hydrogen gas emission. Therefore the battery charger and method described herein can be advantageously used to recharge battery systems including cells with aqueous electrolyte.